I. Field of the Invention
This invention relates to broadcasting methods and broadcasting antenna systems including method and systems of the type utilized in commercial AM broadcast operations.
II. Prior Art and Other Considerations
A variety of communications services which depend upon the propagation of a "ground wave" or "surface wave" along the earth's surface suffer severe interference problems due to the reflection of signals on their own or related frequencies from ionized layers in the earth's upper atmosphere (the ionosphere). These reflected waves are commonly called "sky waves". Among the services suffering from such interference problems are commercial AM broadcast operations, certain types of navigational aids (LORAN, etc.) and 27 MHz. Citizens band communications.
In general, the interfering signals can originate in either the same radiating system as that which generates the desired "ground wave", or from other facilities using the same or related channels (frequencies), located perhaps many hundreds of miles away. In the former case, the interfering signal, being of the same frequency as the desired signal, and bearing the same modulation, can cause severe variations of amplitude and phase of the received signal. This interference, generally known as "selective fading", can seriously limit the usefulness of a facility.
In the latter case, interference commonly termed "co-channel" interference is caused by sources other than the source of the desired "ground wave". Because the interfering signals in this case bear modulation different than that of the desired signal, their disruptive effect can be even more pronounced than the selective fading-type of interference.
Both the selective fading and co-channel types of interference could be reduced if a means were found to reduce that portion of the energy radiated by an antenna which travels outwardly and upwardly to strike the ionosphere, while at the same time permitting the antenna to launch the desired surface wave efficiently. There have been numerous attempts to design such an antenna. Such efforts have generally depended upon achieving some specific distribution of current and phase on a vertical conductor in order to shape the radiation pattern in the vertical plane (that is, to "squeeze" the radiated signal pattern down as close as possible to the horizontal plane).
In the above regard, U.S. Pat. No. 2,064,204 discloses a broadcast antenna comprising a plurality of separate radiating elements inclined with respect to the vertical and lying in a surface of revolution. U.S. Pat. No. 1,683,739 attempts to set up an antenna array which will confine the electromagnetic waves to a comparatively thin layer adjacent to the surface of the earth by superimposing oscillators in a manner to constitute a linear array of sources which are directive in a vertical plane. The Radio Engineer's Handbook describes a system wherein directivity in a vertical plane is combined with a circular pattern in the horizontal plane by arranging short vertical radiators in concentric rings and providing a uniform progressive phase shift between adjacent antennas in the individual rings. Other patents which concern directivity include U.S. Pat. Nos. 2,283,617; 2,283,618; and 2,283,619 to Wilmotte. Another structure which generates strong ground waves while attempting to suppress sky-ward radiation over a limited range of vertical angles is known as the Franklin antenna.
While prior art antenna structures such as the examples given above have been effective at least to a limited extent in curtailing the generation of sky waves radiated in the vertical plane at angles of greater than 45 degrees with respect to the horizontal, prior art structure has not effectively dealt with sky waves radiated in a range of from about 5 degrees or less to 45 degrees with respect to the horizontal. Sky-ward radiation directed in this range of from about 5 degrees to 45 degrees with respect to the horizontal constitutes some of the most bothersome interference.
In view of the foregoing, it is an object of the present invention to provide an antenna structure and broadcasting method wherein sky wave radiation is curtailed through a wide range of vertical angles.
An advantage of the present invention is the provision of an antenna structure which is relatively inexpensive to build and maintain.
A further advantage of the present invention is the provision of method and apparatus which reduces sky wave interference to ground wave signals on the same or related frequencies.
Yet another advantage of the present invention is the provision of method and apparatus wherein the strength of ground wave signals per unit of applied power is increased as compared to currently employed systems.